The present invention relates to a carrier reproduction circuit for a digital broadcasting receiver to receive BS digital broadcasting, and further in particular relates to a carrier reproduction circuit for a digital broadcasting receiver to receive digital modulated waves in which modulated waves as a result of a plurality of modulation systems respectively with different necessary C/N (the ratio of carrier power to noise power) values undergo time-base-multiplexing for transmission.
In the BS digital broadcasting system, there is employed a system in which burst symbol signals enabling reception with a low C/N value are inserted is adopted, added to a hierarchical transmission system in which the digital modulated waves, for example, 8PSK modulated waves, QPSK modulated waves, and BPSK modulated waves all being the primary signals, which are transmitted in a plurality of modulation systems with different necessary C/N values, are combined every specific interval, and in which transmission takes place repeatedly on a frame-by-frame basis. The burst symbol signals are signals having undergone BPSK modulation in the known PN codes.
Moreover, in such a hierarchical transmission system, the frame synchronization pattern as well as the superframe discrimination signals are also patterned in a predetermined fashion and have undergone BPSK modulation. The stable carrier reproduction is implemented by burst-receiving these BPSK-modulated signals during the reception operation at the time when the receiving C/N value is a low C/N value.
However, there used to be a problem with a carrier reproduction circuit in a conventional digital broadcasting receiver, and in such a case where a frequency converter such as an outdoor unit does not sufficiently perform, since the influence of its phase noise hampers stable carrier reproduction and hampers reception with a low C/N value.
The object of the present invention is to provide a carrier reproduction circuit which can implement stable carrier reproduction in reception with a low C/N value.
A carrier reproduction circuit according to the present invention in a digital broadcasting receiver which receives the digital modulated waves which are created by time-base-multiplexing the modulated waves with a plurality of modulation systems for transmission, comprises:
reception phase detection means to detect the demodulated known-pattern reception phases of the reception signals; and
carrier reproduction phase difference detecting means comprising a phase difference table of an absolute phase having one phase convergence point and a phase difference table of the phase rotated from the absolute phase by 180xc2x0 so that any one thereof is selected based on the phase detected by the reception phase detection means, to send out the output based on the phase difference between the phase obtained from the signal point position of the reception signal and the phase convergence point,
wherein carrier reproduction is implemented by controlling the frequency of the reproduced carrier so that the phase obtained from the signal point position coincides with the phase convergence point.
In the carrier reproduction circuit according to the present invention, carrier reproduction is implemented by undergoing the reproduced carrier frequency control so that the reception phase of the demodulated known-pattern reception signal is detected, and based on the detected reception phase, either the phase difference table of absolute phase having one convergence point or the phase difference table of the phase rotated from the absolute phase by 180xc2x0 is selected, and from the selected phase difference table the output based on the phase difference between the phase obtained from the signal point position of the reception signal and the phase convergence point is obtained, and the output based on the phase difference becomes 0, that is, the phase obtained from the signal point position coincides with the phase convergence point. In this case, the demodulated known-pattern reception signal is a BPSK signal, and since the phase difference is obtained with reference to the phase difference table having one phase convergence point, with the BPSK phase difference table the phase difference in excess of xc2x190xc2x0 will not take place, and stable carrier reproduction will be able to be implemented even if the receiving C/N is a low C/N.
The carrier reproduction circuit according to the present invention further comprises carrier reproduction phase difference detecting means with a phase difference table having two phase convergence points in correspondence with the BPSK signals,
wherein during the reception period of the demodulated BPSK signal being a primary signal, the phase difference table having two phase convergence points is selected, and carrier reproduction is implemented by undergoing the reproduced carrier frequency control so that the phase obtained from the signal point position of the BPSK signal being the primary signal comes to or coincides with the phase convergence point closer to the phase.
In the carrier reproduction circuit according to the present invention, during the reception period of the demodulated BPSK signal being a primary signal, the phase difference table having two phase convergence points is selected, and carrier reproduction will be able to be implemented by undergoing the reproduced carrier frequency control so that the phase obtained from the signal point position of the BPSK signal being the primary signal comes to or coincides with the phase convergence point closer to the phase.